There is a reservoir of water that can be kept full without drowning the seeds. When I move my plants out into the morning sun outside, the reservoir of water last through the day, so I don't have to worry about them.I have seen a few YouTube videos on those self-wicking seed trays. They get good reviews. Thanks for the link.
The thing I don't yet understand is that if the wicking mat keeps the plant trays constantly in touch with water, what is the difference between that and just having 1/8- to 1/4-inch of water in a 10X20 tray all the time?
The wicking mat isn't sitting on water, its elevated above the water level, just one end wicks water up to keep the mat moist, so the water level is the bottom of the seed cell and only wicks water up to the seeds when needed.